High throughput darkfield/brightfield wafer inspection system using advanced optical techniques

ABSTRACT

The broadband brightfield/darkfield wafer inspection system provided receives broadband brightfield illumination information via a defect detector, which signals for initiation of darkfield illumination. The defect detector forms a two dimensional histogram of the defect data and a dual mode defect decision algorithm and post processor assess defects. Darkfield radiation is provided by two adjustable height laser beams which illuminate the surface of the wafer from approximately 6 to 39 degrees. Each laser is oriented at an azimuth angle 45 degrees from the orientation of the manhattan geometry on the wafer, and 90 degrees in azimuth from one another. Vertical angular adjustability is provided by modifying cylindrical lens position to compensate for angular mirror change by translating an adjustable mirror, positioning the illumination spot into the sensor field of view, rotating and subsequently moving the cylindrical lens. A brightfield beamsplitter in the system is removable, and preferably replaced with a blank when performing darkfield illumination. Light level control for the system is provided by a dual polarizer first stage. Light exiting from the second polarizer passes through a filter which absorbs a portion of the light and comprises the second stage of light control. The beam then passes through a polarizing beamsplitter. The second channel is further reflected and polarized and both beams thereafter illuminate the substrate.

This application is a continuation of U.S. patent application Ser. No.10/983,078, entitled “High Throughput Brightfield/Darkfield WaferInspection System Using Advanced Optical Techniques,” inventors,Christopher R. Fairley, et al., filed on Nov. 4, 2004 now U.S. Pat. No.7,164,475, which is a continuation of U.S. patent application Ser. No.09/907,295, filed on Jul. 17 2001, now U.S. Pat. No. 6,816,249, which isa continuation of U.S. patent application Ser. No. 08/991,927, entitled“High Throughput Brightfield/Darkfield Wafer Inspection System UsingAdvanced Optical Techniques,” filed on Dec. 16, 1997, now U.S. Pat. No.6,288,780, which is a continuation-in-part of U.S. patent applicationSer. No. 08/884,467, entitled “Optical Inspection of a Specimen UsingMulti-Channel Response from the Specimen,” filed on Jun. 27, 1997, nowU.S. Pat. No. 5,822,055, which is a continuation of U.S. patentapplication Ser. No. 08/489,019, filed on Jun. 6, 1995, now abandoned,all of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the art of optical inspectionof semiconductor wafers, and more specifically to a high throughputbrightfield and darkfield wafer inspection system having imageprocessing redirected from the mechanical and electronics segments ofthe inspection system to the optical domain.

2. Description of the Related Art

Semiconductor wafer inspection techniques have historically utilizedbrightfield illumination, darkfield illumination, or spatial filtering.Brightfield imaging is not generally sensitive to small particles.Brightfield imaging tends to scatter small particles away from thecollecting aperture, thereby resulting in reduced returned energy. Whena particle is small compared to the optical point spread function of thelens and small compared to the digitizing pixel, the brightfield energyfrom the immediate areas surrounding the particle typically contribute alarge amount of energy. The small reduction in returned energy resultingfrom the small particle makes the particle difficult to detect. Further,the small reduction in energy from a small particles often masked out byreflectivity variations from the bright surrounding background such thatsmall particles cannot be detected without numerous false detections.Additionally, if the small particle is on an area of low reflectivity,which may occur for some process layers on wafers and always forreticles, photomasks, and flat panel displays, the resultant backgroundreturn is low and any further reduction due to the presence of aparticle becomes very difficult to detect.

Newer systems utilize broadband brightfield imaging as opposed totraditional monochromatic or narrow band brightfield imaging. Broadbandbrightfield imaging minimizes contrast variations and coherent noisepresent in narrow band brightfield systems, but are not sensitive tosmall particles.

Darkfield imaging is employed to detect small particles on wafers,reticles, photomasks, flat panels, and other specimens. The advantage ofdarkfield imaging is that flat specular areas scatter very little lightback toward the detector, resulting in a dark image. Darkfieldillumination provides a larger pixel-to-defect ratio, permitting fasterinspections for a given defect size and pixel rate. Darkfield imagingalso permits fourier filtering to enhance signal to noise ratios.

Any surface features or objects protruding above the surface of theobject scatter more light toward the detector in darkfield imaging.Darkfield imaging thus produces a dark image except where circuitfeatures, particles, or other irregularities exist. Particles orirregularities are generally assumed to scatter more light than circuitfeatures. However, while this assumption permits a thorough inspectionfor particles on blank and unpatterned specimens, in the presence ofcircuit features a darkfield particle inspection system can only detectlarge particles which protrude above the circuit features. The resultingdetection sensitivity is not satisfactory for advanced VLSI circuitproduction.

While some attempts to improve darkfield performance have beenattempted, such systems tend to have drawbacks, including drawbacksresulting from the very nature of darkfield illumination. For example,while brightfield illumination floods the entire field of view withlight, darkfield illumination is confined to a narrow strip of light.Due to the nature of lasers, the application of light in darkfieldillumination tends to be non-uniform and limits the amount of data whichcan be collected in a particular time period.

Some imaging systems currently available attempt to address problemsassociated with darkfield imaging. One instrument, manufactured byHitachi, uses the polarization characteristics of the scattered light todistinguish between particles and normal circuit features, based on theassumption that particles depolarize light more than circuit featuresduring the scattering process. When circuit features become relativelysmall (less than or on the order of the wavelength of light), thecircuit can depolarize the scattered light as much as the particles. Asa result, only larger particles can be detected without false detectionof small circuit features.

Further, a system employing a combination of a monochromatic darkfieldand a monochromatic brightfield imaging for wafer inspection is poorlyadapted for inspecting Chemical Mechanical Planarized (CMP) wafers,which often have film thickness variations and a grainy texture. Grainytexture, it should be noted, may also be a result of the metal grainstructure of the wafer.

Another attempt to resolve problems associated with darkfield imagingpositions the incoming darkfield illuminators such that the scatteredlight from circuit lines oriented at 0°, 45°, or 90° are minimized. Thismethod is generally effective on circuit lines, but light scatteringfrom corners is still relatively strong. Further, detection sensitivityfor areas having dense circuit patterns must be reduced to avoid thefalse detection of corners.

Prior systems for processing brightfield and darkfield data have reliedon different processing techniques. The system of FIG. 1 illustrates aprior system which performed a full processing of a wafer usingbrightfield imaging followed by darkfield imaging and subsequentprocessing of the wafer. The problem with this mechanization is thatthroughput, or the time to process a single wafer, is generally poor,and it does not have the capability to use the combined results fromboth brightfield and darkfield imaging. As shown in FIG. 1, brightfieldimaging 101 is performed on wafer 103, wherein the brightfield imaginghas tended to be either monochromatic or narrow band imaging. The waferimage is received via sensor 104, which performs TDI, or Time DelayIntegration, and a phase lock loop analog to digital conversion (PLLAD).Data is then directed to input buffer 105, which passes data to defectdetector 107. Defect detector 107 uses delay 106 to perform a die-to-dieor cell-to-cell comparison of the brightfield image processed wafer 103.The results are then passed to post processor 108 where brightfielddefects are determined and passed. The wafer 103 is then illuminatedusing darkfield illumination 102 in the second run, and all subsequentprocesses are performed on the darkfield image. The result of thissecond run is a list of darkfield defects. The typical defect assessmentperformed by the defect detector and the post processor 108 is to set athreshold above which a feature is considered a defect, and only passingbrightfield or darkfield results exceeding such thresholds. This doesnot completely account for the benefits associated with the combinedeffects of using brightfield and darkfield, and the amount of timenecessary to perform all processing for a single wafer can besignificant.

An alternative to the mechanization of FIG. 1 is presented in FIG. 2.The system of FIG. 2 illustrates simultaneous brightfield illumination201 and darkfield illumination 202 of wafer 203. The simultaneousillumination is typically from a single illumination source, and thesystem receives the wafer images using dual TDI and PLLAD sensors 204and 204′. Each sensor 204 and 204′ receives an image of wafer 203 andloads a signal representing that image into input buffer 205 or 205′,such as RAM. From buffer 205 or 205′ the system feeds data to defectdetector 207 where data representing the wafer 203 is compared tosimilar or reference wafer characteristics under the control of delays206 and 206′. Delays 206 and 206′ each provide timing for a die-to-dieor cell-to-cell comparison by defect detector 207. Defect detector 207uses information from both brightfield and darkfield illumination stepsto determine the location of defects on wafer 203. The combined defectlist from defect detector 207 is then evaluated using post processor 208to identify pattern defects and particles.

The drawback in implementing the system illustrated in FIG. 2 is thatindividual TDI and PLLAD sensors 204 and 204′, input buffers 205 and205′, and delays 206 and 206′ are highly sophisticated and expensivecomponents, and the use of two of each such components significantlyincreases the cost of the entire machine. Further, performance of defectdetector 207 and post processor 208 requires that all data be availableand be evaluated at one time, which can cause significant delays andhigh processing costs. For example, it is not unusual to see brightfieldimaging requiring a very short amount of time while darkfield imagingtakes significantly longer. This system also uses monochromatic ornarrowband brightfield imaging, which has a tendency to exhibitundesirable contrast variations and coherent noise problems as discussedabove.

Spatial filtering is another technique used to enhance the detection ofparticles. With plane wave illumination, the intensity distribution atthe back focal plane of a lens is proportional to the Fourier transformof the object. Further, for a repeating pattern, the Fourier transformconsists of an array of light dots. Placement of a filter in the backfocal plane of the lens to block out the repeating light dots permitsfiltering of the repeating circuit pattern and leaves only non-repeatingsignals, such as particles or other defects. The major limitation ofspatial filtering is that only areas having repeating areas or blankareas may be inspected.

There has been little interest in combining brightfield and darkfieldtechniques due to a lack of understanding of the advantages presented bysuch a technique. All of the machines currently available employingmonochromatic brightfield/darkfield imaging use a single light sourcefor both brightfield and darkfield illumination and do not use acombination of brightfield and darkfield images to determine defects.

Microscopes exist on the market today which combine both monochromaticbrightfield and darkfield illumination, and such microscopes have asingle light source and provide both illuminations simultaneously, thusmaking it impossible to separate the brightfield and darkfield results.Such mechanizations simply result in a combined full-sky illumination.

A further limitation of prior systems is that the illumination sourcestend to be fixed in place, which also fixes the ability of the system topick up defects in surfaces or specimens having different physicalproperties. Typically, a video camera is positioned above the specimenand light is applied to the specimen at a predetermined angle. Theapplication of light to a particle tends to scatter the light, which isthen detected by the video camera. If the specimen contains an irregularsurface configuration, such as excess material or a semiconductorpattern, the fixed angle of the light source may not optimally scatterthe applied light, inhibiting the ability to detect defects. Even for awafer having a regular semiconductor pattern, orientation of theillumination source provides a different return when a pattern featureis oriented at 0°, 45°, or 90°. Also, the support mechanisms andcircuitry associated with the light source tend to be large and bulky,thereby impeding the repositioning capability of the light source.

Another problem with brightfield/darkfield imaging is the use of imagingdevices within the same physical space. Components associated withbrightfield imaging are generally used for darkfield imaging as well,and several overlapping components exist when using both forms ofillumination and detection. However, due to the optical, physical, andother characteristics of components used in brightfield/darkfieldimaging, some components tend to provide advantages with one form ofillumination and disadvantages for the other illumination scheme. Theminimization of the disadvantages associated with a form of imagingimproves the ability to detect problems associated with individualspecimens.

Another problem associated with wafer inspection systems is the controlof light level. Control of light level is particularly complex andcritical where a high level of light collection efficiency is desired,and where the gain of the detector is not readily controlled. Priorsystems for providing light level control for wafer inspection includeproviding absorbing glass in the illumination path, and control over theoutput energy of the laser. These systems either do not performsufficiently and/or are too costly or complex to use efficiently.

It is therefore an object of the current invention to provide a systemfor detecting defects on a wafer, the system having the ability todetect defects beyond those detectable using monochromatic or narrowbandbrightfield imaging alone. Inherent in such a system would be theability to minimize contrast variations and coherent noise problems.

It is another object of the current invention to provide a system fordetecting defects which has the ability to detect small particles,including particles having a size smaller than the wavelength of light,with a minimum number of false detections. The system should provide fora minimum number of components to decrease overall cost and provide formaximum throughput of specimens.

It is yet another object of the current invention to provide a systemwhich provides the ability to perform an accurate inspection of ChemicalMechanical Planarized (CMP) wafers, or other specimens having filmthickness variations or grainy textures.

It is still another object of the current invention to provide a systemfor detecting defects on a wafer wherein the system has the ability tooptimize the incidence of light reflected from specimens having varioussurface characteristics.

It is still another object of the current system to provide bothbrightfield and darkfield illumination using a minimum number ofcomponents in a minimum amount of physical space while simultaneouslyminimizing adverse effects associated with brightfield and darkfieldillumination.

It is still another object of the current invention to provide anefficient method or apparatus for light level control in the waferinspection system.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a broadbandbrightfield/darkfield wafer inspection system. Broadband brightfieldillumination illuminates a wafer, and data from this illumination iscaptured by a sensor. The sensor is preferably a TDI sensor having PLLADcapability, but other sensors, such as a non-integrating CCD or linearsensor may be employed. The sensor thereupon loads a signalrepresentative of the image into an input buffer, which feeds data to adefect detector, where the broadband brightfield data from the samplebeing inspected is compared to a similar sample or reference wafer usingtiming control from a delay. The defect detector signals initiation ofdarkfield illumination of the wafer.

The sensor captures illumination resulting from darkfield illuminationand loads a signal representative of the image into the input buffer,which feeds data to the defect detector. Darkfield data in a similarmanner to broadband brightfield data using the delay. Darkfieldillumination data from the defect detector is then passed to postprocessor.

The defect detector signals commencement of the darkfield imaging basedon the type of wafers presented and the expected timing associated withthe wafers. Broadband brightfield and darkfield data does not overlapalong the system path, and the time associated with processing thecombined broadband brightfield and darkfield data is minimized.

The defect detector includes a 2D histogram circuit which forms a twodimensional histogram of the defect data with brightfield differencesplotted on one axis and darkfield differences plotted on the orthogonalaxis. The histogram information is then applied to a dual mode defectdecision algorithm, which sizes and locates defects resulting from thebrightfield and darkfield inspections. The post processor evaluates thequality and importance of the detected defects. Ideally, the dual modedefect decision algorithm and post processor exclude predictablevariations without identifying them as defects, and identifies otherresponses outside an expected range to be defects, and broadbandbrightfield and darkfield data may be combined and used to accomplishthis intent.

Darkfield radiation is provided by two adjustable height laser beams.The laser beams illuminate the surface of the wafer at an angle ofapproximately 6 to approximately 39 degrees. The first laser is orientedat an azimuth angle 45 degrees greater than the orientation of themanhattan geometry on the wafer, and the second laser is oriented at anazimuth angle 45 degrees less than the manhattan geometry on the wafer,or 90 degrees offset from the first laser.

Darkfield illumination within the system accommodates three elevationangles to provide varying ability to illuminate the wafer. At the highgrazing angle setting, 39 degrees, the best sensitivity for low noisewafers such as smooth film and early etch specimens is available. Thelow grazing angle setting, 6 degrees, provides some attenuation of noisefrom the wafer pattern or from wafer roughness. The 20 degree grazingangle illumination is a compromise setting which offers a tradeoffbetween the sensitivity benefit of the 39 degree angle and the noisereduction of the 6 degree angle.

While the elevation grazing angle settings include 6, 20, and 39 degreesettings, the mechanization of the current invention provides for acontinuously variable angular offset, and thus the elevation grazingangle may vary anywhere from approximately five to approximately 45degrees.

The apparatus providing the adjustable angle uses a rotating cylindricallens to control the angular orientation of the laser spot, a pixel sizechanger, and an adjustable mirror. The angle of the adjustable mirror isaltered to change the angle of incidence of each of the lasers on thewafer. The position of the cylindrical lens is modified to compensatefor that change and maintain the elliptical spot in the correct positionrelative to the surface of the wafer and sensor.

The system can compensate for mirror rotation by moving, rotating, ormoving and rotating the cylindrical lens. The preferred method is totranslate the adjustable mirror in the vertical direction, normal to thewafer, to position the illumination spot into the sensor field of view,to rotate the cylindrical lens to properly orient the ellipse, andfinally to move the cylindrical lens to obtain desired ellipticity.

The brightfield beamsplitter provided is removable, and preferablyreplaced with a blank, or glass, when performing darkfield illumination.This allows more light to pass to sensor and permits greater levels ofdetection in darkfield imaging. An alternative method for producing thesame result is to perform brightfield imaging in a selected color lightspectrum and performing darkfield in a different frequency lightspectrum, such as red being selected for brightfield illumination andgreen for darkfield illumination.

Light level control for the system is provided by a dual polarizer firststage, wherein the polarizers are rotated relative to one another tocontrol the intensity of the beam passing through them. The relativerotation of the polarizers provides variation of the beam intensity in acontinuous manner, preferably without varying the polarization of thebeam. Rotation of the second polarizer controls the balance between thetwo output channels. Light exiting from the second polarizer passesthrough a filter, which is preferably a discrete glass filter, and whichabsorbs a portion of the light and comprises the second stage of lightcontrol.

The beam then passes through a polarizing beamsplitter, which dividesthe light into first and second channels. The second channel is furtherreflected and polarized, as needed, and both beams thereafter illuminatethe substrate. Both beams preferably have equal intensity as theyimpinge on the substrate surface.

Other objects, features, and advantages of the present invention willbecome more apparent from a consideration of the following detaileddescription and from the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior system which performed a full processing of awafer using brightfield imaging followed by darkfield imaging;

FIG. 2 is a system using parallel processing of brightfield anddarkfield data;

FIG. 3 presents an overall block diagram of the wafer inspection systemdisclosed herein;

FIG. 4 illustrates the components of the defect detector;

FIG. 5 is a side view schematic of the broadband brightfield/darkfieldwafer inspection system;

FIG. 6 is a vertical view of the orientation of the dual lasers relativeto the wafer as used in darkfield illumination in the current system;

FIG. 7 illustrates the mechanization of the adjustable angle incidenceof the dual laser beam arrangement used for darkfield illumination;

FIG. 8 presents a functional depiction of the components used indarkfield imaging;

FIG. 9 is a typical arrangement of the dual laser illumination system;

FIG. 10 illustrates an expanded view of the brightfield illuminator;

FIG. 11 shows the image computer subsystem architecture;

FIG. 12 presents a diagram of the fourier spot patterns created from atypical array as produced by illumination from two laser beams;

FIG. 13 is another diagram of the fourier spot patterns; and

FIG. 14 illustrates light level control for the system.

DETAILED DESCRIPTION OF THE INVENTION

The inventive system herein combines large pixel DDF (Directional DarkField) illumination with broadband brightfield illumination to enableincreased sensitivity to previous brightfield systems at higherthroughput with reduced image computing power. The system includes adual darkfield illumination module, providing two laser illuminationbeams having adjustable grazing angle, light level, and polarization.

Darkfield imaging collects scattered light from a defect, whilebrightfield imaging collects reflected light. At a given pixel size, avery small (sub-pixel) imaging system averages everything seen in thepixel, including defect plus background. Brightfield imaging uses asmall enough pixel to resolve the edges of the defect and thereby detecta contrast. Darkfield imaging averages everything contained in a pixel,but the background is always black, and even small defects have atendency to scatter large amounts of light. A flat, opaque defect mayscatter very little light in darkfield, but may provide obvious contrastin brightfield. Small, transparent defects may scatter efficiently indarkfield illumination, but may be very difficult, if not impossible, todetect in brightfield. Darkfield imaging is generally useful indetecting defects having specific height, depending upon interactionbetween illumination with the geometry and effects due to transparentlayers on the specimen.

Because darkfield illumination impinges on the wafer at a grazing angle,the darkfield illumination system better discriminates against previouslayer defects. An adjustable grazing angle permits a tradeoff absolutesensitivity for background noise rejection. At generally low grazingangles, the scatter from surface roughness and from the wafer geometryis reduced, permitting a higher signal-to-noise ratio in the presence ofroughness or pattern noise.

Directional darkfield technology provides two distinct advantages: alarger pixel/defect ratio and fourier filtering.

A higher pixel/defect ratio permits inspection of more wafers and thedetection of smaller defects. Image computer cost is proportional topixel rate, and larger pixels permits the inspection of more wafersinspected per unit time. Required pixel rate increases inversely withthe square of pixel size, thereby translating into less expensiveinspection systems, or more wafers inspected. A higher pixel/defectratio provides the ability to detect smaller defects.

The system disclosed herein uses coherent, directional illumination, ora laser with discrete illumination directions. This optical techniqueenables two optical filtering techniques, namely azimuth filtering andfourier filtering. Fourier filtering is discussed and used herein inaccordance with currently pending U.S. patent application Ser. No.08/906,621, to Steve Montesanto, Gershon Perelman, and Rudolf Brunner,entitled “Fourier Filtering Mechanism for Inspecting Wafers”, filed Aug.5, 1997, the entirety of which is incorporated herein by reference.

Azimuth filtering refers to the ability to reject the scatter fromManhattan geometry (straight line geometry parallel to the rectangulardie edges) by bringing the laser illumination 45 degrees in azimuth tothe die edges. The scatter from the Manhattan geometry is not collectedin the imaging system; the straight line Manhattan routes disappearunder the darkfield illumination of the present system. Scatter from theapproximately 45 degree routes are not filtered by the optics of thecurrent invention, and azimuth filtering ceases to operate effectivelyonce the numerical aperture (NA) of the objective exceeds approximately0.707. When the NA of the objective exceeds approximately 0.707, thescatter from the Manhattan geometry falls within the collection cone ofthe imaging system. Wafer layers having considerable roughness on theedges of the geometry will mitigate the effect of azimuth filtering.

The coherent nature of the inventive system described herein providesfor improved optical filtering on the array portion of wafer geometries.Illuminating a coherent array with coherent light results in a coherentdiffraction pattern, some of which is collected by the imaging optics.The diffraction pattern from an array at the wafer appears as an arrayof spots at the fourier plane of the imaging path. By putting adjustablemechanical blockages in the fourier plane, these spots are filtered out,effectively removing the repeating array content from the image. Afourier filtered array image appears devoid of all repeating content,Manhattan or otherwise. The signal to noise ratio in such an array canbe dramatically increased compared to the SNR without the filter.Without the filter, the array may have efficient scattering properties,permitting only relatively low light levels before the TDI sensor,described below, is taken to saturation. With the filter in place, thesystem can completely eradicate the scatter from the array, therebyproviding a large increase in light level, and a resultant improvementin the sensitivity in the array area. This is impossible to attain innon-laser brightfield systems, since such systems have incoherentillumination. The improved sensitivity resulting from filter placementis difficult to duplicate for scanning laser systems, either brightfieldor darkfield, since it requires that the illumination spot be largeenough to illuminate several cells of the repeating geometry.

FIG. 3 illustrates an overall block diagram of the inventive inspectionsystem disclosed herein. Broadband brightfield illumination 301illuminates the wafer 303, and data from this illumination is capturedby sensor 304. Sensor 304 is preferably a TDI sensor having PLLADcapability, but other sensors, such as a non-integrating CCD or linearlaser could be used. The sensor 304 thereupon loads a signalrepresentative of the image into input buffer 305, which may be RAM.Input buffer 305 feeds data to defect detector 306, where the broadbandbrightfield data from the sample being inspected is compared to asimilar sample or reference wafer using control of delay 307. Delay 307provides the timing to allow for a die-to-die or cell-to-cell comparisonby defect detector 306. Defect detector 306 then signals initiation ofdarkfield illumination 302 of wafer 303.

Sensor 304 captures illumination resulting from darkfield illumination302. The sensor 304 loads a signal representative of the image intoinput buffer 305. Input buffer 305 feeds data to defect detector 306,where the darkfield data from the sample being inspected is compared toa similar sample or reference water using control of delay 307. Delay307 provides the timing to allow for a die-to-die or cell-to-cellcomparison by defect detector 306. Darkfield illumination data fromdefect detector 306 is then operated upon with the broadband brightfielddata and the results are passed to post processor 308.

The components of defect detector 306 are illustrated in FIG. 4. Thedigitized image, either brightfield or darkfield, passes to input buffer305, which passes data to delay 307 and also to input buffer 402. Delay307 passes timing information to delay filter 401. Each of the filters401 and 402 are used to preprocess the image data and can be implementedas a 3 by 3 or 5 by 5 pixel digital filter. While the filters could beimplemented as any digital filter having square or rectangulardimensions, including a 4 by 4, 6 by 6, or 7 by 7 pixel filter, the 3 by3 or 5 by 5 pixel digital filter is preferred. The system appliespreprocessed images from filters 401 and 402 to subtractor 403, whereideal brightfield images are compared with the delayed version of thecurrent specimen. The subtractor 403 then may signal to the system thatdarkfield imaging may proceed. Depending on the time required forbrightfield imaging and loading into 2D histogram circuit 404, darkfieldimaging may alternately be initiated at a prior point in the procedure,such as once data is loaded into input buffer 305, or under otheradvantageous timing conditions which would promote overall efficiency.

Based on the type of wafers presented and the expected timing associatedwith the wafers, an indication to commence darkfield processing shouldoccur such that brightfield and darkfield data does not overlap alongthe system path and the time associated with 2D histogram 404 awaitingdarkfield data is minimized. An alternative method for accomplishingthis goal would be to commence darkfield imaging at a predetermined timeafter brightfield imaging is completed.

Darkfield illumination 302 is performed on the wafer 303, which isreceived by sensor 304 and passed to input buffer 305 and delay 307.Darkfield imaging data passes to defect detector 306 from input buffer305 and delay 307 via filter 401 and filter 402, and thereafter passesto subtractor 403 which compares ideal darkfield images to the delayedversion of the subject wafer. The subtractor 403 passes the darkfielddata to 2D histogram circuit 404. 2D histogram circuit 404 forms a twodimensional histogram of the defect data with brightfield differencesplotted on one axis and darkfield differences plotted on the orthogonalaxis. The histogram information is then applied to a dual mode defectdecision algorithm 405.

Dual mode defect decision algorithm 405 sizes and locates defectsresulting from the brightfield and darkfield inspections, while postprocessor 308 evaluates the quality and importance of the detecteddefects. Different methods may be employed by the dual mode defectdecision algorithm 405 and post processor 308 to detect significantdefects. Semiconductor wafers often exhibit surface features such ascontrast variations, grain and grain clusters, or process variationssuch as chemical smear. Each of these anomalies do not generally impactthe performance of a die produced on such a wafer, but can be a concernunder some circumstances. Each of these surface features also has atypical range of brightfield and darkfield readings associatedtherewith. Additionally, noise is associated with system operation, andthis noise can cause variations in brightfield and darkfield differencesignals.

Ideally, the dual mode defect decision algorithm 405 and post processor308 exclude predictable variations without identifying them as defects,and identifies other responses outside an expected range to be defects.In the current system, this may be accomplished in a number of ways. Thepreferred implementation is that for a defect having a magnitudeexceeding a threshold value for either brightfield or darkfieldillumination, the defect is considered of concern and passed from thedual mode defect decision algorithm 405 and post processor 308. Thesystem may evaluate particular characteristics detectable by brightfieldillumination, such as those defects scattering little light, and othercharacteristics detectable by darkfield illumination, such as aheightened irregularity, and base results only on a combination of botheffects. False readings could be reduced by requiring both brightfieldand darkfield readings exceeding particular thresholds. Othermechanizations, depending on the types of defects generally found in thespecimens, may be utilized while still within the course and scope ofthis invention.

Defect detector 306 and, in particular, defect decision algorithm 405,therefore determine whether a defect exists, based on known defectcharacteristics and machine limitations, irrespective of the size orquality of such defect, while post processor 308 determines whether thedefect is relatively significant. The post processor 308 also receivesdata from subtractor 403.

Dual mode post processor 308 can be based on any high performance postprocessor board, such as a Motorola 68040 CPU based VME (Virtual MachineEnvironment) board.

Another view of the system is presented in the simplified schematicdiagram of FIG. 5. Wafer 501 lies on chuck 502 which interacts withtheta brake 503 and ECS head 504. A feedback loop between the drive unit506, drive amplifier 505, and the ECS head 504 provides a uniformmovement of the components and wafer 501 based on a control signalreceived from the optics plate. The wafer is illuminated in brightfieldusing brightfield illuminator 507 having illuminator lens arrangement508, brightfield turning mirror 509, first filter 510, ND (neutraldensity) filter 511, and dual lens arrangement 512 a and 512 b.Broadband brightfield light passes from dual 140 mm lens arrangement 512a and 512 b to brightfield beamsplitter 513, which passes some light andreflects other light through turret turning mirror 521. Light thereuponpasses through objective 522, which contains a condensing lens to focusthe light, and thereupon onto wafer 501. Light is reflected from wafer501 back through objective 522, turret turning mirror 521,brightfield/darkfield beamsplitter arrangement 513, and through 615 nmdichroic mirror 514, variable post mag 515, fourier filter 516, upperturning mirror 517, zoom lens arrangement 518 a and 518 b, and flippingmirror 519 to review camera 520 and sensor 304. Brightfield/darkfieldbeamsplitter arrangement 513 is discussed below.

First filter 510 is preferably an approximately 450-600 nm filter, whiledual lens arrangement 512 a and 512 b is preferably a 140 mm lens, butsimilar components having features outside these ranges may be employedif they produce similar filtering and optical effects.

Under darkfield illumination, adjustable angle laser arrangement 523directs at least one laser beam, and preferably two laser beams over thewafer 501 at a variable angle, where refraction from the wafer passesthrough objective 522, turret turning mirror 521, brightfield/darkfieldbeamsplitter arrangement 513, 615 nm dichroic mirror 514, variable postmag 515, fourier filter 516, upper turning mirror 517, zoom lensarrangement 518 a and 518 b, and flipping mirror 519 to review camera520 and sensor 304.

The system thereby includes both broadband brightfield and darkfieldillumination. Monochromatic darkfield radiation is provided by twoadjustable height laser beams. The laser beams illuminate the surface ofthe wafer at an angle of approximately 5 to approximately 45 degrees. Asshown in FIG. 6, first laser 601 is oriented at an azimuth angleapproximately degrees greater than the orientation of the Manhattangeometry on the wafer 602, and the second laser 603 is oriented at anazimuth angle approximately 45 degrees less than the Manhattan geometryon the wafer 602, or approximately 90 degrees offset from the firstlaser 601. These ranges could conceivably vary while still within thescope of the current invention. For example, slight offsets in theManhattan geometry or placement of lasers 601 and 603 significantlycloser or further apart may produce beneficial effects depending onvarious factors.

The darkfield illumination system depicted in FIG. 5 serves severalpurposes. The system allows control of the elevation angle of the laserbeams incident on the wafer, control over the polarization of the laser,control over the amount of laser power present in the field of view, andcontrol over the shape of the beam within the field of view as afunction of angle and pixel size. Darkfield illumination within thesystem of FIG. 5 accommodates elevation angles between 5 and 45 degrees,with settings for three elevation angles (6, 20, and 39 degrees)available. Darkfield illumination supports various pixel sizes, butpreferably two pixel sizes (2.5 micrometers and 1.25 micrometers), andalso preferably supports three polarization settings (S, P, andcircular).

The laser is preferably a 100 mW frequency double diode-pumped YAG (DPY)operating at a wavelength of 532 nm, but other lasers having similarcapabilities could be used while still within the scope of the presentinvention.

The system of FIG. 5 has continuously variable elevation angles toprovide adjustable illumination of the wafer. At the a high grazingangle, such as an approximately 39 degree setting, the best sensitivityfor low noise wafers such as smooth film and early etch specimens isavailable. In such situations the pattern scatter is low and littlenoise from surface roughness exists. The scattering efficiency of agiven small defect increases as the illumination angle approaches normalincidence. At a low grazing angle, such as the approximately 6 degreesetting, provides some attenuation of noise from the wafer pattern orfrom wafer roughness. This noise attenuation is not necessarily atcomparable expense to the signal from defects on the wafer, so thesignal to noise may be increased on some wafers where pattern andsurface roughness dominate. The elevation effects are due to coherentillumination creating standing waves at the wafer surface, with a node(no field) at the wafer surface at equally spaced heights above it. Asthe illumination angle becomes more grazing, the spacing between nodesincreases, such that surface roughness and patterns close to the wafersurface have low field strengths, and defects higher above the surfaceof the wafer have higher field strengths. The approximately 20 degreegrazing angle setting offers a compromise setting which offers atradeoff between the sensitivity benefit of the approximately 39 degreesetting and the noise reduction of the approximately 6 degree setting.Again, within the scope of the current invention, as described herein,this angle may be altered continuously within the range of approximately5 to approximately 45 degrees.

In order to provide the adjustable angle for the dual lasers 601 and603, the apparatus presented in FIG. 7 is employed. The apparatusincludes a laser 701 to produce laser beam 601 or 603, a rotatingcylindrical lens 702 to control the angular orientation of the laserspot, a pixel size changer 703, and an adjustable mirror 704.

Light from laser 701 is shaped as an elliptical spot by rotatingcylindrical lens 702. Pixel size is controlled by pixel size changer703, while adjustable mirror 704 reflects the laser onto the surface ofthe semiconductor wafer 501. The angle of adjustable mirror 704 isaltered to change the angle of incidence of the beam from each laser 601or 603 on wafer 501. A simple mirror adjustment, without more, changesthe position of the laser spot on the wafer 501, and also changes theshape and orientation of the elliptical spot on the wafer 501. Thisadjustment would cause the sensor 304 to image a portion of the waferwith poor illumination uniformity, or with very little illumination.Thus, the position of the cylindrical lens 702 is modified to compensatefor that change and maintain the elliptical spot in the correct positionrelative to the surface of the wafer 501 and sensor 304.

The system can compensate for mirror rotation by moving cylindrical lens702, rotating cylindrical lens 702, or both moving and rotatingcylindrical lens 702. The preferred method is to translate mirror 704 inthe vertical direction, normal to the wafer 501, to position theillumination spot into the sensor field of view, and to rotatecylindrical lens 702 to properly orient the ellipse, and finally to movethe cylindrical lens 702 to obtain desired ellipticity.

The sensor 304 is preferably a TDI sensor, and the illumination systemshould include subsystems for controlling the intensity and polarizationof the beam, as described herein. TDI sensing is described in detail inU.S. Pat. No. 4,877,326 to Chadwick et al., entitled “Method andApparatus for Optical Inspection of Substrates”, issued Oct. 31, 1989,the entirety of which is incorporated herein by reference. The pixelsize changer 703 may include a set of paired Barlow lenses on a slidehaving a plurality of preset positions.

Beginning with the laser source and following one of the beam paths,FIG. 8 presents a functional depiction of the components used indarkfield imaging. Laser module 801 consists of a 100 mW laser source801 a and a beam expander 801 b for controlling the overall size of thebeam. The preferred beam expander is 6×, but other magnification beamexpanders could be used to control the beam. Analog light level controlmechanism 802 consists of a manually adjusted polarizing beamsplitter802 a and a motorized half wave plate 802 b used in concert as a pair ofcrossed polarizers. Adjustable turning mirror 803 is used for alignment.Switchable beam de-expander 804 is inserted to adjust the illuminationfor review purposes, making the beam wider in X and narrower in Y.Adjustable turning mirror 803 and beam translation plate 805 are usedfor alignment, while polarizing beam splitter 806 splits the beam intotwo paths. Switchable quarter-wave plate 807 is used for circularpolarization, while rotatable half-wave plate 808 is used for S or Ppolarization control. Cylindrical lens 809 maintains twist and focuscontrol, while beam transition plate 810 is used for alignment. Lensarrangement 811, preferably a set of two Barlow lenses, permitadjustment of the beam to the selected darkfield pixel size. The lensarrangement 811 is preferably on a two position slide for selecting fromthe two available pixel sizes, but could be on an N position slide.Pixel size may be continuously varied as opposed to discretelyavailable.

Grazing angle mirror arrangement 812 preferably comprises three grazingangle mirrors to direct the beam onto the wafer in the TDI field ofview. The grazing angle mirror arrangement 812 provides individualmirror adjustability for height, elevation angle, and azimuth angle. Themirrors are each on a three position translation mechanism for selectingeach of the three available angles.

Beam shaping controls control the use of laser power in the field ofview. A typical arrangement of the dual laser illumination system isshown in FIG. 9. First laser 601 and second laser 603 provide laserillumination field 901 within the TDI field of view 902 using laserbeams 903 and 904. Unlike brightfield illumination where the entirefield of view in is flooded with light, the laser illumination indarkfield illumination is confined to a narrow strip, thereby ensuringthat as much light as possible is impinging on the TDI field of view902. The beams are kept narrow in the X direction to maintain the entirebeam in X is within the field of view 902. The beam is kept long in theY direction to provide uniform intensity across the Y dimension of thesensor. Laser power outside the sensor in the Y direction issuperfluous, but the arrangement provides uniform illumination with awell controlled wavefront in the field of view 902. The two beamsemanating from first laser 601 and second laser 603 are shown overlaidin FIG. 9. While the two beams may not overlay precisely, the two beamsmay be seen within the field of view 902. The tight beam dimensioning inthe X direction allows the two beams to wander while still enabling alllaser power in the X direction to be collected by the TDI. Again,darkfield illumination permits collection only if scattering sourcesexist in the field of view 902.

The beam in FIG. 9 is sized for the field of view 802. Two sets ofBarlow lenses, or telescope tubes, exist in the Barlow lens arrangement911 which change the beam size by a factor of two to enable twodarkfield pixel sizes.

The angular orientation and focus or width of the beam in FIG. 9 is afunction of the elevation angle. As elevation angle varies, the beamrotates within the field of view 902 of the TDI. Cylindrical lens 909 isused to correct the angular rotation of the beam, and a focus adjustmenton cylindrical lens 909 provides control for the width of the beam. Pathlength from the cylindrical lens 909 to the wafer changes with elevationangle. Twist and focus settings are controlled for each elevation angleso that the beams are maintained in a vertical orientation independentof the elevation angle selected, and beam position and alignment ismaintained within the TDI field of view 902.

Review camera 520 performs all beam alignment, and the review camera isprealigned to the TDI in a separate alignment operation. The alignmentprocess aligns the beams in the darkfield module such that they passthrough the center of rotation of the cylindrical lenses and reach thelower mirror plugs at an angle normal to the wafer. Lower mirror plugsare thereafter aligned to bring laser beams 601 and 603 to the center ofthe TDI field of view at an accurate elevation angle. Azimuth angle isthen adjusted for one of the beams 601 or 603 to match the azimuth angleof the other laser beam such that the fourier patterns align and thefourier filter 516 can be utilized by both beams.

Darkfield illumination parameters visible to the user/operator includeangle of incidence, polarization, and light level. Angle is a selectionof the position of the six mirror rack arrangement, described below, orthe lower turning mirrors, which turn the beam from the verticaldirection to the grazing incidence on the wafer, as well as adjustmentof the cylindrical lens twist and focus to maintain beam orientation andwidth within the TDI field of view 902.

Polarization includes selection of the position of the switchablequarter-wave plate 807 in or out of the beam path and the position ofthe rotatable half-wave plate 808. Switchable quarter-wave plate 807transforms linear to circular polarization, while rotatable half-waveplate 808 rotates incoming polarization, and has rotary positions for S,P, and C polarizations. The C position is halfway between the S and Ppositions, and the C position must match C polarizations betweenmachines.

The effect of polarization is most pronounced as the illumination anglebecomes more grazing S polarization is most commonly used, particularlyat low grazing on rough, opaque wafers, where noise reduction isdesired. On wafers having transparent surface films, the scatteringbehavior of surface defects tends to be influenced by the localthickness of the film. The C, or circular, setting permits a combinationof the S and P polarizations to illuminate the wafer and for the twopolarizations to average, with less resultant influence from the filmthickness. The P polarization produces an anti-node at the wafer surfacerather than the node produced with the S polarization.

The system also provides variable illumination adjustability, includingadjustability into saturation. The system can specify illuminationrather than TDI gray levels, which is useful when local processes causethe light level algorithms to produce increased sensitivity variationover the simple light fixed level. The system further sets sight levelscausing saturation in the TDI image. Some features will not subtractwell whether or not they saturate, so the illumination strategy is touse as much light as possible and not critically evaluate corners butinstead examine the specimen for defects in the open areas. Thisrequires the sensor 304 to be allowed to go deep into saturation withoutblooming.

Light level adjustment is performed in two stages, fine setting andcoarse setting. Fine setting, or analog light level, uses a set ofadjustable cross polarizers, while coarse setting, or digital tightlevel, switches in different ND filters. Manually adjusted polarizingbeamsplitter 802 a of analog light level control mechanism 802 ispositioned on a manually rotatable mount set at alignment time toapportion laser power between the two beam paths. Laser light passinginto the analog light level control assembly is linearly polarized, andthe polarization is rotated by the motorized half wave plate 802 b. Theresultant linear polarization can range continuously from parallel withthe output from the polarizing beamsplitter 802 a to orthogonal to it.

The laser light emitted from the beamsplitter 802 a has fixedpolarization as the output polarizer does not move, and laser powervaries sinusoidally from maximum to minimum as the half wave platerotates. An extinction ratio, representing the ratio from maximum tominimum of approximately 300 to 1 is typically achieved by the analoglight level mechanism 802. The output polarization is at approximately45 degrees to the plane of the darkfield optics module such thatdownstream polarizing beamsplitters at 90 degrees to the plane of themodule can receive half the laser power for each beam path.

The digital light level mechanism consists of a set of three ND filters,preferably two filters plus a clear aperture, providing additionalattenuation of zero, 10×, or 100×. This additional attenuation isrequired to assure that the full dynamic range and resolution of lightsetting is achievable, preferably a light level range of 1000 to 1 and aresolution of one percent at any light level setting.

A laser power sensor is positioned downstream from the last polarizingbeamsplitter in the optics train. The laser power sensor measures afixed proportion of the light allowed into the two beam paths, and istherefore a measure of the light level setting from the two light levelmechanisms. The laser power sensor is also used to perform a calibrationoperation on the light level mechanisms.

Calibration of the darkfield illumination system utilizes the laserpower sensor, which provides a reading proportional to the illuminationin the two beam paths. The calibration consists of rotating the analoglight level mechanism 802 to determine light level as a function ofangle, then switching the ND filters to find the exact attenuation as afunction of each filter. The resultant data is used to create a mappingof percent light level to positions of the analog and digital NDswithout gaps or discontinuities.

The system further includes 2 segment Segment Automated Thresholding™,or SAT™. SAT™ automatically separates the digitized wafer image intodifferent regions called “segments” based on process noise andbrightness. Peak sensitivity is achieved by assigning separatethresholds to each segment of the image rather than a single thresholdfor the entire image. Optimal thresholds are automatically determinedfor each segment based upon the process variability which exists on theinspected wafer. The ability of SAT™ to adapt to changing processconditions provides greater sensitivity to be achieved and maintainedwafer-to-wafer and lot-to-lot without nuisance defects.

2 segment SAT™ uses high sensitivity in the extreme upper left corner ofthe mean/range histogram where the mean and range are low. This area isassociated with small defects on a background, where the goal is tooptimize the sensitivity in the clear areas and evaluate any informationavailable in the rest of the image.

The brightfield radiation employed in the system of FIG. 5 is broadbandflood illumination. The broadband radiation optically averages intensityvariations, including interference effects caused by thicknessvariations in transparent films. Averaging interference effectsincreases signal-to-noise ratios for higher sensitivity to almost alldefect types, including CMP related defects. The result is animprovement in defect detection sensitivity, particularly on wafershaving extreme color variations. Broadband flood illumination alsoprovides faster setup times and improved throughput in manyapplications.

The system may include an anti-vignetting aperture to improve thecollection uniformity of the imaging system, thereby reducing the needto correct the image electronically.

FIG. 10 is an expanded view of brightfield illuminator 507. Backingmirror 1001 reflects light from arc lamp 1002 through condenser 1003.Upper turning mirror 1004 then passes the light to lower turning mirror1005, which reflects the light to illuminator lens arrangement 508,which includes 70 mm lens 1006, condenser 1007, and 21 mm lens 1008.

The image computer subsystem architecture 1100 is presented in FIG. 11.Similar components are available in the KLA-Tencor Corporation Model2135 unit, with the exception being those components having particularshading. Diagonal shading indicates those components having hardwarechanges from the 2135, and vertical shading represents a componenthaving embedded code changes. VIF 1101 includes changes inde-multiplexing circuitry on the sensor motherboard at the front end ofthe image computer. The mass memory board 1102, or MM2, is modified toadd memory and allow reprocessing of edge die for large pixel darkfieldinspections. The mass memory board 1102 memory is organized as odd andeven data banks, making it impossible to issue data for two odd or twoeven dice simultaneously for edge die evaluation. The systemmodifications add shadow banks of reversed odd and even order, so evendie go into the even bank and also to a shadow odd bank that can be usedduring edge die reprocessing. The amount of mass memory is thereforedoubled for the current system over the 2135. The 2135 uses four imagecomputers each having 64 MB of mass memory on each MM2 board, for atotal of 256 MB of mass memory. The current system uses a single imagecomputer with a total of 512 MB of mass memory on its only MM2 board.Programmable logic changes accommodate the additional memory banks andallow edge reprocessing.

The FFA 1103, or full field alignment system, firmware accommodates analignment kernal change from 32×512 pixels to 32×2048 pixels. The XYinterpolator (XYI) boards 1104 and 1105 require additional memory asthey include a pipeline delay to allow the FFA to calculate alignmenterrors before the data is corrected by the interpolators. A single pairof XYI boards 1104 and 1105 is utilized in the system to delay andinterpolate a swath of data. The 2135 uses four pairs of boards toaccomplish this task. The delay capacity on the new boards 1104 and 1105are therefore increased.

TDI sensing provides a wider range of data collection over previoussystems, which could typically collect only a single narrow band of dataat one time. TDI provides a wider overall collection area. Pixel sizefor the TDI sensor 304 of the present system is preferably a 16micrometer square, but other dimensions may be employed. The smallerpixel reduces the sensor's contribution to the noise budget of thesystem. The TDI sensor also provides anti blooming capability to permituse of saturating light levels for darkfield inspections. The TDI sensoralso provides a current output to accommodate designing for highbandwidth outputs. The system preferably runs at 200 Mpx, but canaccommodate at least up to approximately 800 Mpx. The TDI sensor 304provides more pixels in the shift direction, or X direction, to enablelonger integration and a wider illumination field for brightfield atapproximately 800 Mpx. The feature is not used in brightfieldillumination in the present system, but the additional pixels are usedin darkfield illumination to accommodate beam wander without losingillumination efficiency. The additional pixels cannot be used inbrightfield since the amount of distortion is proportional to fieldsize, and an increase in the illumination field results in a blurrierTDI image. Brightfield illumination is limited within the system using afield stop in the illumination train to limit the illumination topreferably 256 TDI pixels in the X direction. The number of TDI pixelsmay vary to a greater or lesser than 16 by 16 range.

Fourier filtering comprises an adjustable array of wire blockages whichfilters repetitive features in the Y-direction of the TDI image. Thefourier filter has adjustable pitch for spacing variance and adjustableoffset for position variance to accommodate various fourier patterns orsemiconductor arrangements. The formula for the pitch of the diffractionspots in the fourier plane is:F _(i)=(λ*L _(f))/(M _(obj) *M _(pc) *A _(i))where F_(i) is the pitch of the spots at the fourier plane in mm, λ isthe wavelength of the laser in micrometers, L_(f) is the reference focallength of the objective lenses in mm, which is preferably 200 mm,M_(obj) is the objective lens magnification, which is preferably 10 to20 for the darkfield pixels, M_(pc) is the power changer magnification(preferably 0.5 or 1), and A_(i) is the cell size of the array in the Ydirection. Again, these dimensions may vary while maintaining adequateperformance and still be within the scope of the invention.

The fourier filter removes the repeating content of an array area butleaves non-repetitive features. This permits clear transmission ofdefects without the clutter associated with repeating patterns, as apoint source defect is spread throughout the fourier plane, so most ofthe energy passes through the mechanical blockages. A typical fourierfilter reading is shown in FIGS. 12 and 13. In FIG. 12, a diagram of thefourier spot patterns 1201 created from a typical array is shown asproduced by the illumination of two laser beams, each providing acoherent, collimated beam of light. A beam 1202 of incident laser lightα hits the wafer producing the α diffraction orders 1203 and α specularreflection 1204. A beam 1205 of incident laser light β strikes the waferproducing the B diffraction orders 1206 and β specular reflection 1207.A fourier plane aperture 1208 samples the diffraction patterns for α andβ.

Size as shown in FIGS. 12 and 13 represent intensity of the diffractionspots. The spots do not actually vary in size as shown, but do vary inintensity. The two shadings distinguish between diffraction spots fromthe two incident beams.

FIG. 13 illustrates a diagram of the fourier spot patterns 1301 createdfrom the array is shown produced by the illumination of two laser beams.FIG. 13 is a diagram of the diffraction spots and filter blockages atillumination elevation angle theta. A beam 1302 of incident laser lighta strikes the wafer producing the α diffraction orders 1363, which areblocked by wires or springs acting as filter blockages 1304. A beam 1305of incident laser light B strikes the wafer producing the β diffractionorders 1306 also blocked by filter blockages 1304. A fourier planeaperture 1307 samples the diffraction pattern for α and β.

Using fourier filtering, the scatter from surface roughness causes noeffect, so if an array of rough metal or rough polysilicon is fourierfiltered, the resultant image is of unpatterned rough metal orunpatterned rough polysilicon. An array of noisy material produces anoisy image, so fourier filtering becomes less effective as the arraybecomes less regular or more rough. The fourier filtering removesspatial frequencies from the image content, affecting the non-repetitivefeatures passing through the filter. To create an image of apoint-source defect, all spatial frequencies are required. With certainfrequencies removed, the point defect becomes a repeating pattern in theimage.

Sidelobes caused by a defect in an array caused by the fourier filterare visible in the darkfield image. The effect of these sidelobes isaccounted for and corrected by the system.

The fourier filter 516 operates only in the Y direction and has limitedrange. The two illumination beams of the system each create a fourierpattern. The spacing of the fourier spots changes inversely with cellsize on the wafer and illumination wavelength. Spacing does not changewith grazing angle, but the pattern does move or translate as a wholewith variations in grazing angle. The fourier pattern for the twoillumination beams moves in tandem (in the same direction in Y) when thegrazing angle is changed, so a single set of blockages is used for bothbeams. The fourier patterns also move in the X direction, specificallyin opposite directions, with changes in grazing angle. Straight-lineblockages in the X direction allow any amount of X direction movement ofthe spots without impairing filtering ability. The apparent spacing ofthe blockages is also influenced by optics magnification, with spacingdecreasing as magnification increases. Since filter spacing has alimited range, some cell sizes may be filtered at one magnification butnot another. Since fourier filter spacing can be made only so small, amaximum array size exists that can be filtered. The maximum cell size ispreferably 20 micrometers for the 10×1 optics configuration, 10micrometers for the 20×1 configuration, and 5 micrometers for the 20×2configuration, but these values may be altered to maintain adequateoverall performance. The fourier filter is set such that settingstranslate from one machine to another, and where feasible from oneoptical configuration to another.

The fourier image is developed at the pupil plane behind the powerchargers in the imaging path. Using the review camera 520, the fourierplane can be imaged by inserting the Bertrand lens into the review path.The Bertrand lens can be accessed from the review user interface or fromsuperdiags. The diffraction spots and the position of the fourier filterblockages may be monitored by imaging the fourier plane, and thistechnique can be used to manually adjust the filter to block aparticular wafer pattern.

As the system of FIG. 5 utilizes several redundant components inperforming both brightfield and darkfield imaging, some components mayexist which are beneficial to one form of illumination and not theother. The brightfield beamsplitter 513 provides a benefit to thebroadband brightfield imaging described herein, but degrades performanceof darkfield imaging. Thus the brightfield beamsplitter is removable,and preferably replaced with a blank, or glass, when performingdarkfield illumination. This allows more light to pass to sensor 304 andpermits greater levels of detection in darkfield imaging. An alternativemethod for producing the same result is to perform brightfield imagingin a selected color spectrum and performing darkfield in a differentfrequency color spectrum. For example, if brightfield illuminator 507 orany component along the path prior to the beamsplitter used red light,the darkfield imaging components, such as the lasers 601 and 603 woulduse green light, where red light and green light have differentfrequencies. Such an implementation removes the negative effectsassociated with the brightfield beamsplitter 513 when used for darkfieldimaging.

The brightfield optics are thus similar to those used in the KLA-TencorCorporation Model 2135. The autofocus system is identical in the presentsystem. The brightfield illumination system has an additional shutterand field stop. The imaging path preferably includes a new 20× objective515, and the objective 515 allows high angle laser incidence. A higheror lower magnification objective may be used while still within thescope of the invention. The brightfield beamsplitter 513, as discussedherein, is removable to improve darkfield collection efficiency.

Power changers which provide for a larger fourier plane are used in thepresent system. The increased power changers yield the magnification ofthe tubes scaling back by a factor of two so that the size of thefourier plane scales upward by a factor of two. The power changers inthe present system thus preferably have magnifications of approximately0.5× and 1×, but other values are available, such as between the rangesof 0.25× and 1×. The magnifications are compensated for in the zoom magand TDI pixel size. The tube improvements include optimizing the qualityof the fourier image to minimize distortions so that the fourierblockages are as small as possible. The system further has the abilityto match the fourier plane focus between the approximately 0.5× and 1×tubes, so that the same fourier position is used for both tubes. The0.5× power changer is used for darkfield inspections, but the 1× tube isalso used for darkfield rearview, so the fourier filter may be used withboth power changers. Fourier focus matching requires an axial focusadjustment for the 0.5× power changer. Again, other magnitude componentsmay be used while maintaining performance and still be within the scopeof the invention.

The system further includes a Y mirror having a slightly larger mirroraperture and a small displacement of the mirror assembly to provide roomfor the fourier filter. The zoom assembly corrects magnificationdifferences between the present system and other system imaging optics.Again, the TDI sensor produces lower noise and anti-blooming capability,and the review optics of the new system match the present systemmagnifications to other systems.

The light level control for the system is illustrated in FIG. 14. Laser601 or 603 generates a laser beam, which passes through a firstpolarizer 1401 and a second polarizer 1402. The polarizers are rotatedrelative to one another to control the intensity of the beam passingthrough them. The combination of polarizers 1401 and 1402 comprises thefirst stage of light level control. The relative rotation of thepolarizers 1401 and 1402 provides variation of the beam intensity in acontinuous manner, preferably without varying the polarization of thebeam.

Rotation of polarizer 1402 controls the balance between the two outputchannels. The channels are preferably balanced to provide two laserlines of similar intensity on the surface of the substrate. The linesmay be oriented at an angle relative to one another such that they willprovide adequate illumination for various substrate topographies.

Light exiting from polarizer 1402 passes through filter 1403, which ispreferably a discrete glass filter. Filter 1403 absorbs a portion of thelight. The filter may be selected from a set of filters included withinthe system, and may filter the light intensity by 100 to 1, 10 to 1, 1to 1, or some other quantity. The filter 1403 comprises the second stageof light control.

The beam then passes through a polarizing beamsplitter 1404, whichdivides the light into first and second channels. The second channel isfurther reflected and polarized, as needed, as represented in FIG. 14 aselement 1405, and both beams thereafter illuminate the substrate. Bothbeams preferably have equal intensity as they impinge on the substratesurface.

A laser power sensor may be used to monitor laser power delivered to thewafer 501. This laser power sensor may be used in a feedback loop tocontrol the system light level. As noted above, the laser power sensoris positioned downstream from the polarizing beamsplitter, and the laserpower sensor measures leakage from the beamsplitter. Leakage measurementthereby measures a fixed portion of the light allowed into the twochannels. Signals from the laser power sensor can be used to controlboth stages of the light level control. The system is initiallycalibrated to verify the functionality of the feedback loop and makeappropriate adjustments.

While the invention has been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

1. A device for controlling light level generated by a light generatingdevice used with a specimen inspection system, comprising: a first lightlevel control stage comprising a first polarizer and a second polarizer,wherein the second polarizer is configured to be rotated with respect tothe first polarizer to control beam intensity for light beams passingthrough the first light level control stage and enable control ofreceived beam intensity in a continuous manner without substantiallyvarying polarization of light beams passing therethrough; a firstpolarizing beamsplitter configured to divide light energy received fromthe first light level control stage into first and second channels, saidfirst channel representing first light energy; and a second polarizingbeamsplitter configured to receive the second channel from the firstpolarizing beamsplitter and transmit second light energy; wherein firstlight energy and second light energy impinge on a surface of a specimenat substantially equal intensity.
 2. The device of claim 1, wherein thelight generating device comprises a laser.
 3. The device of claim 2,further comprising a laser power sensor used to monitor laser powerdelivered to the specimen.
 4. The device of claim 1, further comprisinga second light level control stage comprising a filter positionedbetween the first light level control stage and the first polarizingbeamsplitter.
 5. The device of claim 4, wherein the filter comprises adiscrete glass filter.
 6. The device of claim 1, wherein relativeorientation of the first polarizing beamsplitter and the secondpolarizing beamsplitter may be altered to provide illumination based onspecimen topography.
 7. A method for controlling light level generatedby a light generating device used with a specimen inspection system,comprising: providing light energy; receiving the light energy in afirst stage comprising at least two polarizers configured to be rotatedrelatively to control beam intensity for light beams passing through thefirst stage, thereby enabling control of received beam intensity in acontinuous manner without substantially varying polarization of lightbeams passing therethrough and providing controlled light energy;dividing the controlled light energy received from the first stage intofirst and second channels, said first channel representing first lightenergy, said second channel forming second light energy; and causingfirst light energy and second light energy impinge on a specimen surfaceat substantially equal intensity.
 8. The method of claim 7, whereinlight energy comprises laser light energy.
 9. The method of claim 8,further comprising monitoring laser power delivered to the specimenusing a laser power sensor.
 10. The method of claim 7, furthercomprising deflecting the second channel into second light energy usinga polarizing beamsplitter.
 11. The method of claim 7, further comprisingfiltering the controlled light energy before dividing the controlledenergy.
 12. The method of claim 11, wherein the filtering occurs using adiscrete glass filter.
 13. The method of claim 7, further comprisingmonitoring light energy delivered to the specimen.
 14. The method ofclaim 7, wherein relative orientation of first light energy and secondlight energy impingement may be altered to provide illumination based onspecimen topography.
 15. An apparatus configured to control a level oflight generated by a light generating device used with a specimeninspection system, comprising: a first stage configured to receive lightenergy from the light generating device, the first stage comprising afirst polarizer and a second polarizer, wherein the second polarizer isconfigured to be rotated with respect to the first polarizer tocontinuously control light energy beam intensity; a first polarizingbeamsplitter configured to divide light energy received from the firststage into first and second channels, said first channel representingfirst light energy; and a second polarizing beamsplitter configured toreceive the second channel from the first polarizing beamsplitter andtransmit second light energy; wherein first light energy and secondlight energy impinge on a surface of a specimen at substantially equalintensity.
 16. The apparatus of claim 15, wherein the light generatingdevice comprises a laser.
 17. The apparatus of claim 16, furthercomprising a laser power sensor used to monitor laser power delivered tothe specimen.
 18. The apparatus of claim 15, further comprising a secondstage comprising a filter positioned between the first stage and thefirst polarizing beamsplitter.
 19. The apparatus of claim 18, whereinthe filter comprises a discrete glass filter.
 20. The apparatus of claim15, wherein relative orientation of the first polarizing beamsplitterand the second polarizing beamsplitter may be altered to provideillumination based on specimen topography.